1. Field of the Invention
The present invention relates to a ball bearing comprising a surface-coated metal ball. More particularly, the present invention relates to a bearing suitable for hard disk drive (hereinafter referred to as xe2x80x9cHDDxe2x80x9d) spindle motor device or the like comprising rolling elements which have a hard ceramic coating film or diamond-like carbon coating film formed on the surface thereof to exhibit an enhanced fretting resistance and prevent the unintentional release of preload due to temperature change.
2. Description of the Related Art
Despite its shorter history than other industries, the computer-related industries are making an extremely rapid technical innovation. In particular, HDD industry has introduced new techniques to make successive development of new compact models having a smaller power loss requirement, a high response and a high precision. Under these circumstances, bearing performance corresponding to these properties has been required.
Referring to bearing for HDD device, for example, a small-sized deep groove ball bearing is often used for spindle motor shown in FIG. 6 and swing arm motor shown in FIG. 7. The ball bearing 1 for spindle motor is used to allow a cup-shaped flange 2 on which a magnetic disc (not shown) is mounted to be smoothly driven rotatively around a shaft 4 provided standing on a base 3 by a motor M. Thus, the ball bearing 1 is required to have remarkably excellent running performance and acoustic properties. The ball bearing 1 for swing arm is used to allow a swing arm 7 to be swung smoothly around a shaft 9 provided on a base 8. The swing arm 7 allows a head 6 to be accessed and positioned on the effective area on a magnetic disc D. A preload is applied to these ball bearings 1 at room temperature to enhance the shaft supporting rigidity. However, since the motor for HDD device is required to have a reduced size, constant-pressure preload process, which requires some space, cannot be employed. Therefore, constant-position preload process is employed in which an inner race 1n and an outer race 1g of two ball bearings are fixed to shafts 4, 9 and the inner wall of a flange 2 or sleeve 10 as a rotary body, respectively, with an adhesive while being pressured by applying a load downward.
As the material of ball bearing to be used in the foregoing HDD device, there is often used SUJ2 (JIS), which is a high carbon chromium bearing steel, SUS440C (JIS) which is a martensitic stainless steel, 0.7Cxe2x80x9413Cr stainless steel, or the like. These steel materials are hardened and tempered to obtain desired hardness or wear resistance. Thus, steel materials, the hardness of which has HRC of 58 to 64 are used.
However, the ball bearing 1 for HDD device is subject to adverse effect on acoustic properties or vibration properties due to fretting wear developed by the microvibration of the rotary portions (flange 2, sleeve 10) in the rotating direction during the transportation of the device. Fretting wear takes place on the balls B in the bearing 1. As a countermeasure against fretting abrasion, the use of ceramics as bearing ball B has begun. This is because the surface properties, hardness, mechanical strength, chemical stability and wear resistance of ball made of ceramics are better than that of ball made of steel such as bearing steel.
The ball made of ceramics has excellent surface properties but has a linear thermal expansion coefficient which is 70% smaller than that made of steel and a modulus of a longitudinal elasticity which is 50% greater than that made of steel. Thus, when the temperature rises during the use of the device, ball bearings employing constant-position preload process such as one for motor for HDD device is subject to great. change in the maximum contact stress between ball and its rolling surface such as to cause a great drop in the bearing rigidity (preload). Thus, in an extreme case; so-called release of preload, i.e., zeroing of preload during use can take place.
Referring to the possibility of release of preload, its mechanism will be studied hereinafter.
Referring to elastic deformation and stress on the contact area at which the ball comes in rolling contact with the rolling surface, Herz""s theory of elastic contact can be applied. In general, as shown in FIG. 8A, when two objects I and II which are elastic materials having a smooth surface come in contact with each other, main planes of curvature 1 and 2 crossing each other at right angle in symmetrical planes exist in the vicinity of the contact point. As shown in FIG. 8B, the object I has radii rI1 and rI2 of main curvature in the section of main planes of curvature, respectively. The object II has radii rII1 and rII2 of main curvature in the section of main planes of curvature, respectively. The reciprocal of these radii rI1, rI2, rII1 and rII2 of main curvature (distinguished by signs + and xe2x88x92, which means that the curvature is convex or concave, respectively) are defined to be xcfx81I1, xcfx81I2, xcfx81II1 and xcfx81II2, respectively.
Formed at the contact area is a contact ellipsoid A having two radii (major radius a and minor radius b) crossing each. other.
Supposing that when a vertical load Q is applied to the contact ellipsoid A, the maximum contact stress acting on the. center of the contact ellipsoid A is "sgr"max and the amount by which the elastic objects I and II displacement each other is xcex4, "sgr"max and xcex4 can be given by the following equations, respectively.
"sgr"max=3/2xcfx80xc2x71/xcexcxcexdxc2x73{square root over ( )}[1/(3/2)2{(1xe2x88x921/m2I)/EI+(1xe2x88x921/m2II)/EII}2xc2x7(xcexa3xcfx81)2xc2x7Q]
xcex4=3/4xc2x72k/xcfx80xcexcxc2x73{square root over ( )}[2/3xc2x7{(1xe2x88x921/m2I)/EI+(1xe2x88x921/m2II)/EII}2xcexa3xcfx81xc2x7Q2]
A further study will be made by applying the foregoing equations to the contact of ball B with the rolling surface in the outer race 1g and the rolling surface in the inner race 1n in a deep groove ball bearing 1 as shown in FIG. 9. It is supposed that the ball B is made of ceramics and the outer race 1g and inner race 1n are made of steel.
The ceramics has a modulus of longitudinal elasticity E1 of 313.6 GPa, a Poisson""s ratio m1 of 10/2.7, a linear thermal expansion coefficient A1 of 3.2xc3x9710xe2x88x926/xc2x0 C. (same as that of silicon nitride Si3N4) and a thermal conductivity B1 of 10.8 W/(mxc2x7k).
The steel has a modulus of longitudinal elasticity EII of 207.8 GPa, a Poisson""s ratio mII of 10/3, a linear thermal expansion coefficient AII of 11.8xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity BII of 76 W/(mxc2x7k). In the linear thermal expansion coefficient, an average of minimum and maximum are used among that of representative steel materials, i.e., martensite stainless steel (10.1xc3x9710xe2x88x926), bearing steel SUJ2 (12.5xc3x9710xe2x88x926), middle, low carbon steel (13.5xc3x9710xe2x88x926), to which the invention is applied.
The maximum contact stress "sgr"max and the displacemnet xcex4 are represented by the following equations:
"sgr"max=210xc3x97(1/xcexcxcexd)3{xcexa3xcfx81)2Q}xe2x80x83xe2x80x83(1)
xcex4=(1.13/103)(2K/xcfx80xcexc)3{xcexa3xcfx81Q2}xe2x80x83xe2x80x83(2)
wherein xcexcxcexd and 2K/xcfx80xcexc are a function of xcfx81.
Supposing that in FIG. 9 the diameter of ball B is d, the radius of curvature of the curved surface of groove in the section of the race groove on the outer race 1g including the bearing axis is r0, the radius of curvature of the curved surface of groove in the section crossing the bearing axis at right angle is R0, the radius of curvature of the curved surface of groove in the section of the race groove on the inner race 1n including the bearing axis is ri, and the radius of curvature of the curved surface of groove in the section crossing the axis 1 at right angle is Ri, the sum of main curvatures xcexa3xcfx81=xcfx81I1+xcfx81I2+xcfx81II1+xcfx81II2 is represented by the following equations:
For the contact of inner race with ball,
xcexa3xcfx81=4/d+(1/Ri)xe2x88x92(1/ri)xe2x80x83xe2x80x83(3)
For the contact of outer race with ball,
xcfx81xcfx81=4/d+(1/R0)xe2x88x92(1/r0)xe2x80x83xe2x80x83(4)
The auxiliary variable cos xcfx84=|(xcfx81I1xe2x88x92xcfx81I2)+(xcfx81II1xe2x88x92xcfx81II2)|/xcexa3xcfx81 is represented by the following equations:
For the contact of inner race with ball,
cos xcfx84={(1/ri)+(1/Ri)}/xcexa3xcfx81xe2x80x83xe2x80x83(5)
For the contact of outer race with ball,
cos xcfx84={(1/r0)+(1/R0)}/xcexa3xcfx81xe2x80x83xe2x80x83(6)
Since the spring constant k is dQ/dxcex4,
k=1/{(1.13/103)(2K/xcfx80xcexc)}xc2x73/2xc2x73(Q/xcexa3xcfx81)xe2x80x83xe2x80x83(7)
Supposing that the ball having a diameter of 2 mm in bearing number B5-39 (inner diameter: 5 mm; outer diameter: 13 mm; width: 3 mm), which is a bearing for HDD, is made of ceramics, numerical values will be calculated as follows:
d=2.0 mm
ri=1.07 mm; r0=1.07 mm;
Ri=3.5 mm; R0=5.50 mm+xcex4s=5.508 mm (in which clearance 67s is 16 xcexcm)
By substituting these values for the equations (3) to (6) to calculate xcexa3xcfx81 and cos xcfx84, the values of xcexc, xcexd, xcexcxcexd, and 2K/xcfx80xcexc are then determined. The results are set forth in Table 1.
When the maximum contact stress "sgr"max and displacement amount (deformation amount) xcex4 of the inner race and outer race are determined from the equations (1), (2) and (7), the following results are obtained:
For inner race,                                                                                           σ                  maxi                                =                                  1.78                  xc3x97                                      10                    2                                    ⁢                                      xe2x80x83                                    ⁢                                      Q                    3                                                                                                                                            δ                  i                                =                                  8.43                  xc3x97                                      10                                          -                      4                                                        ⁢                                      xe2x80x83                                    ⁢                                                            Q                      2                                        3                                                                                                                                            For                  ⁢                                      xe2x80x83                                    ⁢                  outer                  ⁢                                      xe2x80x83                                    ⁢                  race                                ,                                                                                                          σ                  maxo                                =                                  1.47                  xc3x97                                      10                    2                                    ⁢                                      xe2x80x83                                    ⁢                                      Q                    3                                                                                                                                            δ                  o                                =                                  8.05                  xc3x97                                      10                                          -                      4                                                        ⁢                                      xe2x80x83                                    ⁢                                                            Q                      2                                        3                                                                                      ]                            (        8        )            
From the equation (8), the relationship kxe2x88x9d3Qxe2x88x9d"sgr"maxi is established.
Referring next to the case where both the ball and the outer and inner races are made of steel, the maximum contact stress "sgr"max and the displacement xcex4 of steel ball are represented by the following equations:
"sgr"max=187xc3x97(1/xcexcxcexd)3{(xcexa3xcfx81)2Q}
xcex4=(1.28/103)(2K/xcfx80xcexc)3{(xcexa3xcfx81Q2}
Therefore, the comparison of the load Qxe2x80x2 and displacement xcex4cxe2x80x2 of steel ball with the load Q and approach xcex4c of ceramics ball with respect to the same maximum contact stress "sgr"xe2x80x2maxi gives the following results:                                                                                           Q                  xe2x80x2                                =                                                                                                    (                                                  210                          /                          187                                                )                                            3                                        ⁢                    Q                                    =                                      1.41                    ⁢                                          xe2x80x83                                        ⁢                    Q                                                                                                                                            δ                  c                  xe2x80x2                                =                                                                            (                                              1.28                        /                        1.13                                            )                                        ⁢                                                                  {                                                                                                            (                                                                                                Q                                  xe2x80x2                                                                /                                Q                                                            )                                                        2                                                    ⁢                                                      δ                            c                                                                          }                                            3                                                        =                                      1.42                    ⁢                                          xe2x80x83                                        ⁢                                          δ                      c                                                                                                          ]                            (        9        )            
wherein xcex4c=xcex4i+xcex40 
It can be seen that the load Qxe2x80x2 and displacement amount xcex4cxe2x80x2 of steel ball are each 1.4 times that of ceramics ball.
The load and displacement amount of ceramics ball and steel ball were determined with the value of "sgr"maxi being 0.20, 0.39, 0.59, 0.78, 0.98, 1.18 and 1.37 GPa. The results are plotted in FIG. 10. In general, when an ordinary preload is added, the value of "sgr"max is 0.98 GPa. At this time, the amount of elastic deformation (displacement amount) xcex4c of the contact point of ceramics ball is as considerably small as 0.522 xcexcm as determined from FIG. 10.
Ceramics ball and steel ball will be compared herein after in the effect of temperature difference on bearing preload.
The difference AA in linear thermal expansion coefficient between steel and ceramics is represented by the following equation:
xcex94A=11.8xc3x9710xe2x88x926/xc2x0 C. (steel)xe2x88x923.2xc3x9710xe2x88x926/xc2x0 C. (ceramics)=8.6xc3x9710xe2x88x926/xc2x0 C.
The contact rigidity of ball with inner raceway surface is proportional to its maximum contact stress "sgr"maxi. When the amount xcex4c of elastic deformation of the contact point is 0.2 xcexcm or less, the value of contact rigidity shows a sudden drop. Supposing that the diameter of ball d is 2 mm, the calculation of temperature change xcex94T corresponding to the difference xcex94xcex4c of 0.2 xcexcm in the amount of elastic deformation (displacement amount) between steel and ceramics gives the following results:
xcex94T=xcex94xcex4c/xcex94Axc2x7ball diameter=0.2/(8.6xc3x9710xe2x88x926xc3x972xc3x97103)=11.6(xc2x0 C.)
Accordingly, the elastic deformation 67 c=0.522 xcexcm of ceramics ball (see FIG. 10) at the foregoing general maximum contact stress "sgr"max=0.98 GPa corresponds to temperature difference of 30.3xc2x0 C. In other words, when the temperature difference between during the predetermination of preload and during the use of bearing reaches 30.3xc2x0 C., ceramics ball loses preload to reduce "sgr"max to 0, causing the release of preload.
In case of the combination of ceramics ball and steel inner and outer races, there is a difference in linear thermal expansion coefficient therebetween. Therefore, the temperature difference xcex94T causes an expansion difference of ball diameter d and inner and outer races, to thereby change a bearing clearance. The change xcex94d is represented by the following equation:
xcex94d=xcex94Axc3x97xcex94Txc3x97d=8.6xc3x9710xe2x88x926xc3x97xcex94Txc3x97dxe2x80x83xe2x80x83(10)
For example, when the maximum contact stress "sgr"maxi is 0.98 GPa at d of 2 mm, the elastic deformation xcex4c of ceramics ball is 0.52 xcexcm.
Accordingly,
(a) Supposing that the temperature during assembly is 20xc2x0 C. and the working temperature is 80xc2x0 C., xcex94T is 60xc2x0 C., giving the following change:
xcex94d=8.6xc3x9710xe2x88x926xc3x9760xc3x972.0xc3x97103 xcexcm=xe2x88x921.032 xcexcm
Since the condition under which the release of preload takes place is xcex94=xcex4c+xcex94d less than 0, preload is released in this case.
(b) Supposing that the temperature during assembly is 60xc2x0 C. and the working temperature is 80xc2x0 C., xcex94T is 20xc2x0 C., giving the following change:
xcex94d=8.6xc3x9710xe2x88x926xc3x9720xc3x972.0xc3x97103 xcexcm=xe2x88x920.344 xcexcm
Since xcex94is 0.18 xcexcm, preload is not released. However, since the maximum contact stress "sgr"max is approximately 0.59 Gpa, when the elastic deformation 67c of ceramics ball is about 0.18 xcexcm as shown in FIG. 10, the rigidity is reduced by 40% from that at the initial "sgr"max value (=0.98 GPa).
(c) Supposing that the temperature during assembly is 60xc2x0 C. and the temperature during the transport of bearing is as low as 0xc2x0 C. to 20xc2x0 C., the temperature change xcex94T is from xe2x88x9260xc2x0 C. to xe2x88x9240xc2x0 C., giving the following change xcex94d of from 0.69 to 1.0 xcexcm. Accordingly, xcex94(=xcex4c+xcex94d) is from 1.21 to 1.52 xcexcm, giving a tendency toward the rise in preload.
Supposing that xcex94 is 66 c, the maximum contact stress "sgr"max will be determined as follows:
From the equation (8), the amount of elastic deformation xcex4c of ceramics ball is given by the following equation:
xcex4c=xcex4i+xcex40=(8.05xc3x9710xe2x88x924+8.43xc3x9710xe2x88x924)3Q2=16.48xc3x9710xe2x88x924Q2/3
Accordingly,
Q1/3=(xcex4cxc2x7104/16.48)=24.633xcex4c
Similarly, from the equation (8),                                                                         σ                maxi                            =                              xe2x80x83                            ⁢                              1.78                xc3x97                                  10                  2                                ⁢                                  xe2x80x83                                ⁢                                  Q                  3                                                                                                        =                              xe2x80x83                            ⁢                                                1.78                  xc3x97                                      10                    2                                    xc3x97                  24.633                  ⁢                                      xe2x80x83                                    ⁢                                                            δ                      c                                                                      =                                  43.85                  xc3x97                                      10                    2                                    ⁢                                                            δ                      c                                                                                                                              (        11        )            
In other words, when xcex4c is 1.21xc3x9710xe2x88x923 mm, "sgr"maxi is 1.49 GPa, and when xcex4c is 1.52xc3x9710xe2x88x923 mm, "sgr"maxi is 1.68 GPa. Thus, when the temperature lowers from the value during assembly, preload rises, causing the maximum contact stress "sgr"maxi to rise.
As can be seen in the foregoing calculation, when the difference in linear thermal expansion coefficient between ceramics and steel causes preload to rise or fall, the bearing working temperature exceeds the assembly temperature, causing the release of preload, or falls below the assembly temperature, causing a rise of preload. Thus, when the bearing rigidity changes, the natural frequency of HDD spindle motor, for example, changes to coincide with a specific frequency vibration generated by the combination of a large number of geometric. error components of the balls and rolling surfaces on the inner and outer races in ball bearing, increasing the possibility of resonance. Further, taking into account the change with time between the starting of rotation and the stabilization of temperature, resonance inevitably appears.
Further, since ceramics which are usually used in rolling bearing, such as silicon nitride, zirconia and alumina, are insulating materials, it is disadvantageous in that they electrostatically attract particles, giving an adverse effect on the acoustic properties of the device.
The present invention has been worked out paying attention to these problems of the related art. An object of the invention is to provide a ball bearing which can give solution to problems such as fretting wear, release of preload and deterioration of acoustic properties attributed to electrostaticity at the same time.
In order to accomplish the foregoing object of the invention, according to a first aspect of the invention, there is provided a rolling bearing comprising an inner race, an outer race and a plurality of rolling elements as constituents, characterized in that the inner race, outer race and rolling elements are made of a steel having a linear thermal expansion coefficient of from 10.1xc3x9710xe2x88x926 to 13.5xc3x9710xe2x88x926, both inclusive, and the steelball has a hard coating film having a thickness of from 0.5 xcexcm to 2.5 xcexcm, both inclusive, which is harder than the material of the rolling elements. Further, the hard coating film can be made of ceramic, an electrically-conductive ceramic, or a diamond-like carbon. Moreover, the base material of the rolling element is a martensite stainless steel. In addition, the hard coating film consists of an interlayer and a surface layers
In addition, according to the invention, there is provided a bearing device comprising as constituents two or more bearings having an inner race fitted in an outer race apart from each other in the axial direction and a housing in which the bearing outer race is fitted, the bearing inner race and outer race being relatively rotatable via the rolling elements, a positioning preload being applied in the axial direction, characterized in that there is used the above rolling bearing. Further, according to the invention, the axial direction may be the direction of gravity.
In addition, according to the invention, there is provided a magnetic recording apparatus comprising the above bearing device, wherein said rolling bearing is made of an electrically-conductive ceramic.
According to a second aspect of the invention, the ball in a ball bearing is a steel ball having almost the same linear thermal expansion coefficient as that of the bearing race in the ball bearing, the metal ball being coated with a ceramics material. Examples of the metallic material constituting the ball and bearing ring include bearing steel such as SUJ2, case hardening steel such as SUS440C, 0.7Cxe2x80x9413Cr martensite stainless steel and SCr(JIS), various alloy steels having secondary hardenability, high speed steel such as SKH(JIS) and special abrasion-resistant undeformable steel such as SKD(JIS). These steel materials have a linear thermal expansion coefficient of from 10.1xc3x9710xe2x88x926 to 13.5xc3x9710xe2x88x926/xc2x0 C.
The ball in the ball bearing of the invention has a surface coating film made of ceramics and thus has a high surface hardness and mechanical strength and an excellent chemical stability and doesn""t form any alloy when brought into contact with the bearing ring. Therefore, the ball in the ball bearing of the invention has the same fretting resistance as that of ceramics ball. Further, the main ball body is made of a metal such as steel and thus has the same linear thermal expansion coefficient as that of the inner and outer races. Thus, the main ball body undergoes no change in the maximum stress at the contact area with the rise in the temperature of the entire bearing. Accordingly, even when the temperature rises during use, any phenomenon such as drop of bearing rigidity and release of preload does not occur.
With respect to transient phenomenon with the temperature change, the ball of the invention has a smaller longitudinal elastic modulus and a greater thermal conductivity than ceramics ball to advantage.
Further, the rolling bearing according to the invention comprises rolling elements the rolling surface of which is made of an electrically-conductive ceramics. Accordingly, the rolling elements cannot be electrostatically charged and thus cannot attract particles, providing the bearing with excellent acoustic properties.
Moreover, in the rolling bearing according to the invention, at least one rolling surface of the rolling elements is formed by a low wear diamond-like carbon (hereinafter referred to as xe2x80x9cDLCxe2x80x9d). Thus, when used in HDD spindle motor or actuator, the rolling bearing according to the invention can be prevented from alloying with the race surface with which it comes in contact. Accordingly, the rolling surface of the rolling elements can be worn little, giving an excellent fretting resistance. At the same time, the rolling bearing according to the invention shows a prolonged life, a low torque and little torque variation under rotational and vibrational conditions during use.